A multipoint control system, which connects a plurality of points to each other, and enables participants in a remote site to participates in and hold a conference, is widely utilized, for example, as a remote conference system.
One example of the multipoint control system is disclosed in Patent document 1 (hereinafter, referred to as a prior art 1). The multipoint control system, as shown in FIG. 74, includes terminals 9000, 9001, and 9002 being dispersively arranged in respective points, and a multipoint control unit (MCU) 9005 for controlling data exchange between the terminals. The multipoint control unit 9005 mixes signals outputted from respective terminals, and distributes it to all terminals. The multipoint control unit 9005 excludes only the signal outputted from the terminal of a distribution destination when mixing the signals. For example, the signal being distributed to the terminal 9000 is one obtained by mixing the signals outputted from the terminals 9001 and 9002.
A configuration example of the multipoint control unit 9005 is shown in FIG. 75. While an example of connecting three points is shown in FIG. 75, the multipoint control unit 9005 can be configured to connect the points of which the number is arbitrary. In FIG. 75, transmission signals received from the terminals arranged in a first point to a third point are outputted to decoding units 9020, 9030, and 9040 via an input terminal, and decoded in the decoding units 9020, 9030, and 9040, respectively. The decoded signals are further outputted to mixing units 9021, 9031, and 9041. The mixing unit 9021 mixes the decoded signals coming from the second point and the third point, generates a mixed signal being sent to the first point, and outputs the mixed signal to an encoding unit 9022. The mixing unit 9031 mixes the decoded signals coming from the first point and the third point, generates the mixed signal being sent to the second point, and outputs the mixed signal to an encoding unit 9032. The mixing unit 9041 mixes the decoded signals coming from the first point and the second point, generates the mixed signal being sent to the third point, and outputs the mixed signal to an encoding unit 9042. Each of the encoding units 9022, 9032, and 9042 encodes the mixed signal, and outputs it to an output terminal via each terminal. Additionally, not each of the mixing units 9021, 9031, and 9041 mixes a plurality of the signals simply, but also pre-decided various media processes (an image process, a sound process, a data process, etc.) are applicable hereto.
A first configuration example of the terminals 9000, 9001, and 9002 is shown in FIG. 74. Additionally, each of these terminals can assume an identical configuration, so the configuration of only the terminal 9000 is shown. From now on, the terminal 9000 is exemplified for explanation. The terminal 9000 is configured of a transmission unit 9006 including a noise suppression unit 9010 and an encoding unit 9011, and a receiving unit 9007 including a decoding unit 9012. The input signal is inputted into the noise suppression unit 9010 of the transmission unit 9006 via the input terminal. For example, with general mobile telephones, the signal caught by a microphone (a microphone signal) becomes an input signal. The microphone signal is configured of desired sound and background noise (hereinafter, referred to as noise), and the noise suppression unit 9010 suppresses only the noise with the desired sound kept as it stands, if possible, and outputs it as noise-suppressed sound to the encoding unit 9011. The encoding unit 9011 encodes the noise-suppressed sound outputted from the noise suppression unit 9010 based upon the encoding technique such as CELP. After the encoded information is outputted via the output terminal, and subjected to modulation/amplitude etc., it is outputted to a transmission path. That is, the transmission unit 9006 performs the process such as sound encoding after performing the noise suppression process, and output the signal to the transmission path. The receiving unit 9007, after demodulating and digitalizing the signal received from the transmission path, outputs it to the decoding unit 9012. The decoding unit 9012 decodes the inputted signal, and outputs it as an output signal. The output signal is inputted into a loudspeaker, and reproduced as an audible signal.
The noise suppression unit 9010, which is generally known as a noise suppressor (noise suppression system), suppresses the noise superposed upon a desired sound signal. The noise suppressor operates so as to suppress the noise coexisting in the desired sound signal by, as a rule, estimating a power spectrum of a noise component by employing the input signal converted into a frequency region, and subtracting this estimated power spectrum from the input signal. Successively estimating the power spectrum of the noise component makes it possible to apply the noise suppressor also for the suppression of non-constant noise. One example of the technology of the noise suppressor is disclosed in Patent document 2 (hereinafter, referred to as a prior art 2).
In addition, the technology of curtailing an arithmetic quantity for suppression of the noise is disclosed in Non-patent document 1 (hereinafter, referred to as a prior art 3).
These techniques are identical to each other in a basic operation. That is, the above technique is for converting the input signal into the frequency region with a linear transform, extracting an amplitude component, and calculating a suppression coefficient frequency component by frequency component. Combining a product of the above suppression coefficient and amplitude in each frequency component, and a phase of each frequency component, and subjecting it to an inverse conversion allows a noise-suppressed output to be obtained. At this time, the suppression coefficient is a value ranging from zero to one (1), the output is completely suppressed, namely, the output is zero when the suppression coefficient is zero, and the input is outputted as it stands without suppression when the suppression coefficient is one (1).
As a second configuration example of the terminals 9000, 9001, and 9002, the case that the noise suppression unit 9010 is excluded from a first configuration example of FIG. 74 and does not exist can be listed. This configuration is equivalent to not only the case that the terminal is not provided with the noise suppression unit 9010, but also the case that a user switches off its function, or the case that a suppression degree of the noise suppression unit 9010 is not sufficient. In such a terminal, the noise etc. getting mixed with the desired sound signal is not suppressed sufficiently, and is transmitted to other terminals as it stands. In this case, the noise gets mixed with the mixed signal that a participant of the conference hears, and hence, a sound quality declines. This causes a problem that the participant hears important phrases by mistake, or his/her fatigue augments due to long-time utilization of the above terminal. The similar problem arises in the case that a suppression degree of the noise suppression unit 9010 is not sufficient, or the function of the noise suppression unit 9010 is invalidly set even though the terminal of the first configuration example having the noise suppression unit 9010 is employed.
Herein, as a rule, residual noise, which stays as a result of being not suppressed, and distortion of the noise-suppressed sound, which is outputted, are in a relation of trade-off. Reducing the residual noise leads to an increase in the distortion, and reducing the distortion leads to an increase in the residual noise. In the prior art 1, the noise suppression unit of the transmission unit adjusts a balance between the residual noise and the distortion, namely, controls the desired sound and the noise.
Patent document 1: JP-P2000-83229A
Patent document 2: JP-P2002-204175A
Non-patent document 1: PROCEEDINGS OF ICASSP, Vol. 1, pp. 473-476, May 2006